Subtropical Mode Water in the Northwestern Pacific Subtropical Mode Water in the Northwestern Pacific

Subtropical Mode Water in the Northwestern Pacific

  • 期刊名字:青岛海洋大学学报
  • 文件大小:383kb
  • 论文作者:PAN Aijun,LIU Qinyu
  • 作者单位:Physical Oceanography Lab. & Ocean-Atmosphere Interaction and Climate Lab.
  • 更新时间:2020-07-08
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论文简介

Journal of Ocean University of Qingdao (Oceanic and Coastal Sea Ressarch)ISSN 1671-2463, October 31, 2003, Vol.2, No.2, pp.134-140http:/ www. ouC. edu. .n/xbyw6/E-imail: xbywh@ mail. ouC. edn. onSubtropical Mode Water in the Northwestern PacificPAN Aijun', LIU QinyuP7 APhysical Oceanography Lab. & Ocean. Atmosphere Interaction and Climate Lab, Ocean Universiry of China,Qingdao 266003, P. R. China(Received March 13, 2003; accepted August 28, 2003)Abstract Based on the in situ XBT and other data sets, by analyzing the seasonal cycle of the mixed layer depth (MLD)and using the conservative potential vorticity (PV) as a tool, a clear description of the formation process of the North Pacif-ic Subtropical Mode Water (NPSTMW) is presented for explaining the well known ‘ Stommel Demon '。The forming ofNPSTMW reflects well the ventilation process of the isotherms of the permanent thermocline. The formation process can bedivided into the ' ventilation ' phase and the‘formation' phase. In the first phase (October- March), with large heat losses atthe sea surface from October, the mixed layer deepens and correspondingly, the water mass with low PV emerges and sinks.After continual cooling from October to March, the mixed layer reaches its maximum value (> 300 m) in March. Then, inthe sccond phase ( April-June), the mixed layer shoals rapidly from April, a large part of the low PV water mass is shelteredfrom further air- sea interaction by the emerging seasonal thermocline, and thus forms new NPSTMW. Further analysis indi-cates that the formation region of warm NPSTMW (17- 18C) is limited between 140*- 150'E, while the relatively coldNPSTMW (16- 17C) originates in a wider longitude range (140* - 170'E) .Climate features of NPSTMW are presented with the use of climatological Levitus (1994a, b) dataset. It is shown thatNPSTMW lies in the region of (130*- 170'E, 22*- 34'N) with core temperature ranging from about 16- 19C and potentialdensity around 25 - 25.801. NPSTMW has a three. dimensional structure lying below the seasonal thermocline ( about 100 mdeep) and reaches almost to 350 m depths.Key words North Pacific Subtropical Mode Water (NPSTMW); ventilation process; mixed layer depth (MLD); potentialvorticity (PV)Number ISSN 16712463<2003)02- 134-07part of the mixed layer is covered over by the overly-ing seasonal pycnocline and subducted into the main1 Introductionthermocline.Mode water (Worthington, 1959) is a special typeSince Masuzawa (1969) first detected the Northof water mass characterized by its vertical homogene-Pacific Subtropical Mode Water (NPSTMW) in theity of temperature, salinity and low potential vorticity North Pacific western basin, there have been some(PV) and lies between the seasonal thermocline andstudies in this field. NPSTMW will be advected aroundthe permanent thermocline below. Mode water' s for-the Subtropical Gyre after its formation. Observationsmation is a local enhancement of ventilation process,have provided enough evidences that the formation re-which happens in the outcropping region of isothermsgime of the NPSTMW is only one small part of theof the ventilated thermocline. It can be easily detectedwhole NPSTMW distribution area. Suga et al. (1989)for its vertical temperature or potential vorticity min-examined NPSTMW in the repeatable hydrographicima in the water column. As to why the mode water section along the 137°E section made in winter (Janu-takes only the late winter properties of water massesary) and in Summer (June-July) and showed the sum.from the sea surface instead of showing the same sea-mertime NPSTMW is advected along the inner path ofsonal cycle like sea surface temperature (SST), Stom-the Kuroshio Countercurrent from the relatively west-mel (1979) gave an acceptable explanation now known ern part of the formation area, taking approximately 4as the‘Stommel demon' : basically, the mixed layer' smonths,while the wintertime NPSTMW advecteddensity and depth formed by wintertime deep conve-" 中国煤化ely eastern part ofction reaches its maximum in later winter, since inthemonths. Usingearly spring the mixed layer rapidly shoals, and the deeptransp:ph data, Bingham( 19921HCNM H Greding orNPSTMW and has put forward a pattern of‘ differential* Corresponding author. Tel:0086-532- 2032556E- mail: iceking@ouc. edu. cnspreading' with the help of low PV, whereby NPST-PAN A.J. et al: Subtropical Mode Water in the Northwestern Pacific135MW formed in the western part is advected to thewest after its formation, while that formed farther thermal dynamic characteristics at the air- sea interface.east is advected farther to the south or east after for-The Levitus (1994a,b) dataset has 1°x1° (latXmation. Suga and Hanawa (1995) further studied thelon) horizontal resolution and represents the results ofcorrelations between NPSTMW and the Kuroshio and objective analysis of all historical temperature profilesfound the advection of NPSTMW depends largely on available at the National Oceanographic Data Centerthe variation of the Kuroshio pathways. During the (NODC/NOAA) through December 1993. It invalvesnon-large meander period, the recirculation system has the water column from surface to about 5500 m depths.a single anticyclonic gyre centered near (30°N, 137E) In the present study, we just take advantage of theand the intense Kuroshio Countercurrent will advect upper 14 levels: 0, 10, 20, 30, 50, 75, 125, 150, 200,NPSTMW formed off the Kuroshio Extension, or east250, 300, 400 and 500 m and interpolate the prime da-of 140°E, to the west of 137°E, south of Honshu within ta to every 5 m in depth using a spline function. Cli-about 4 months. Heavier NPSTMW formed farther mate features of NPSTMW are given by this dataset;east is advected along the outer path, taking several but it represents a long-term mean state, so, for bet-months more. During the large meander period, the ter depiction of the seasonal cycle of NPSTMW forma-recirculation system is divided into two anticyclonic tion we use the more acceptable in situ XBT datasetgyres and for the non-closure of the external stream- below.line, that is, the weak Kuroshio Countercurrent, noThe monthly in situ expendable bathythermographsubstantial westward advection across the 140°E sec- (XBT) data is released by the National Oceanographiction occurs, while minor advection of NPSTMW along Data Center, covering the years from 1955 to 1998,the outer pathway can occur .with2° x 5° (latX lon) horizontal grid at standardThe brief review above shows that up to date there depths of 0, 20, 40, 60, 80, 120, 160, 200, 240, 300,is no clear description on the formation process of and 400 m. Also, XBT data is interpolated to everyNPSTMW from observational data with proper con-5 m in depth using a spline function.sideration of the seasonal cycle of atmospheric forcing,which is crucial for our understanding of the air-sea in-2.2 Defining NPSTMWteraction process. In this paper, we try to solve thisUntil now, there are two kinds of definition ofquestion and hope to provide a clear description on the‘Mode Water': Bingham (1992) defined it as a layerformation process of NPSTMW by analyzing the sea-of vertically homogeneous water found between thesonal cycle of the net heat flux from Comprehensiveseasonal pycnocline and the lower permanent pycno-Ocean- Atmosphere Data Set (COADS) dataset andcline, while Qiu et al. (1995) thought only when thethe mixed layer depth (MLD) from the in situ XBTdeep part of the mixed layer has detrained into thedataset.The paper is arranged as follows: in section 2 wepermanent thermocline could it be called ‘ Mode Waintroduce the data and the definition of MLD and PV.ter'. In this paper, for convenience to study the for-The climate features of NPSTMW are given in Sectionmation of NPSTMW, Bingham (1992)”s definition is3. In section 4 we emphasize the formation process ofadopted. Considering one seasonal cycle, when theNPSTMW by fully considering the atmospheric condi-water mass with low PV along with the deep part oftion and the seasonal cycle of MLD. Conclusions andthe mixed layer in March reaches its maximum depth,with the rapidly shoaling of the mixed layer in earlydiscussions are given in section 5.spring, this vertically homogeneous part of water masswould be sheltered from further air- sea interaction by2 Data and Analysis Methodthe reermerging seasonal thermocline and form the‘NPSTMW'2.1 DataThree kinds of datasets are used in present study:2.3 Definition of Mixed Layer Depth (MLD)COADS, XBT and Levitus (1994a, b) .COADS data was produced at University of Wiscon-By scrutinizing the vertical structure of tempera-sin- Milwaukee(UWM) by Arlindo M. da Silva and ture, salinity in the water column, we can reasonablyChristine C. Young in cllaboration with Syd Levitus identifly a layer with relative homogeneity, which isfrom NOAA/ National Oceanographic Data Center.referred to as‘ mixed layer' (Suga and Hanawa, 1990).The UWM/COADS Data Set contains raw and objec-As a product of ocean-atmosphere interaction in thetively analyzed fields of surface marine anomalies of upper ocean, it has been verified to play a dominantflux of heat, momentum, and fresh water along with中国煤化工,- of mass, heat, mo-other related parameters. Net heat flux data used in I:tween the ocean andthis paper is taken from COADS,which is monthly thYHCN M H G atmospheric forcingdata with 1° X 1° (latX lon) grid covering the period significantlty intluences the upper ocean circulation pat-of 1950-1993. It provides us a general view of the sea-tern and thermal structure. The mixed layer depth136Journal of Ocean University of Qingdao2003, Vol.2, No.2used in this paper is defined to be the depth with the tect the existence of NPSTMW by scrutinizing the PVtemperature of (SST-1C),which is the same as minima of the water column. In Fig.2 we get the cli-used by Hanawa and Hoshino (1988) and Suga and mate features of NPSTMW using the long term meanHanawa (1990).Levitus ( 1994 a, b) dataset. Generally, the NPST-MW lies in the subsurface upper-ocean below 80 m to2.4 Potential Vorticity Definition350 m depths with vertical temperature gradient lowerPotential Vorticity (PV) is a good tracer to studythan 1.5C (100m) ' (Fig.2c),which shows thatthe mode water spreading pathways due to its conser-NPSTMW is a large type of vertical homogeneous wa-vativeness. In this paper we use three PV proxies toter mass as one thermostad. Also, potential tempera-represent the cold, conservative water mass. The first.ture and potential density of NPSTMW are in a limit-one is: PV = f/H,where H represents the verticaled band, for instance, potential temperature is be-separation between two isothermal surfaces, and f istween 16-19C (PV<3X 10°m's') from Fig.2 bthe Coriolis parameter. This proxy is used to examineand potential density is around 25- 25.8a。(PV<2.5Xthe lateral structure of PV on selected isotherms. The10~ "°m~'s") in Fig2c.second one is to represent PV in the high. resolution ver-tical section, defined as:PV= f(aT/aZ), which iscalculated at each point in the 5 m interpolated verticalprofile. The third one is based on the Levitus (1994a,b) temperature and salinity dataset: PV = (flp)●6-.-(Oσg/Oz),which is used to find the climatological4..characteristics of NPSTMW.123 Climate Features of NPSTMW(a)PV defined as Pi-fiBy scrutinizing the PV minima (PV= f/H) of the23.5water column,we can find the corresponding coretemperature of the long-term mean NPSTMW, which20is shown in Fig.1. It is shown that the long-termmean NPSTMW locates between 130*- 170°E, southof the Kuroshio and Kuroshio Extension and north of2the Subtropical Countercurrent (STCC- 20°N) andhave a slight northeastward slant with its core temper-ature corresponding to PV minimum from 16C to2618C. The western part of NPSTMW is relativelywarmer (above 18C) in contrast to its eastern part26.5(b) PV based on penil densit(below 18 C ). Examining the March SST, we may0.015 0.03__ 0.045 0.06see that NPSTMW is formed south off Japan and dis-tributes far wider beyond its formation region.101 200300................40.0.....(C) Temperature gradientFig.2 Scatter diagram for all profiles in the region of 140*-1而ww170'E, 25'-32"N (data from Levitus, (1994a, b))Fig1 March SST (CI -2C) and long- term mean NPST-4 Formation Process of NPSTMWMW based on core temperature by scrutinizing the verticalThere have heep manv resparrhes on NPSMW sinceminimum of PV (shaded) in the North Pacific (data fromLevius, 1994日)its fi中国煤化工: (1969). Exam-For its vertical homogeneity in temperature andples aMYH2 Bingham (1992),SugaCN MH G.da and HanaWadensity, mode water is also taken as‘thermostad' or (1997) and so on, which enrich our knowledge great-‘pycnostad' (Suga et al, 1997), thus we could de- ly on the spatial pattern and temporal variations ofPAN A.J. et al: Subtropical Mode Water in the Northwestern Pacific137NPSTMW. However, on the formation of NPSTMW of net heat flux and MLD, and based on this analysiswe have only one explanation- ‘Stommel Demonto present a clear picture on the NPSTMW formation. .which seems to reflect that we have not paid enoughApril and Oetober are the transition period for windattention on this fundamental problem. Even the stress in the North Pacific. From October the north-‘Stommel Demon ' only explains the formation process west monsoon strengthens, the dry, cold air from Si-of NPSTMW theoretically, while lacking a detailedberia begins to take large amount of heat away fromdescription on it from data analysis. In this paper, we the sea surface (Fig.3), and consequently, the MLDjust want to tackle this problem using in situ XBT data- deepens (Fig.4). With more heat losses from the seaset and hope to present a clear view on the formation surface (October- March), MLD deepens continuouslyprocess of NPSTMW with proper consideration of at- and reaches its maximum value in March over the Ku-mospheric forcing: wind stress and net heat flux .roshio and Kuroshio Extension current. Also, it isThe effects of wind are twofold. First, wind stress noted that there is a lag of MLD deepening with netdrives the upper layer Ekman flow; its contribution toheat flux. From April to May, wind stress changes itsthe mode water formation is still unknown and will direction, the cold, dry northwest winds turns to benot be considered. Secondly, wind stress causes water replaced by the warm, wet southeast winds. Corre-mass convergence and induces Ekman pumping, which spondingly, the ocean begins to gain heat. At thisdeepens MLD about 30ma'. This slow process, com- time, we find there is sharp shoaling in the MLD westpared with the rapid process of deepening and shoaling of 140°E from more than 300 m in March to almostof MLD in wintertime and early spring, can also be100m in April. The MLD east of 140E changes moreneglected. Thus, direct effects of wind stress in deep-sharply in May, from more than 300 m in April to onlyening the mixed layer are not emphasized in this pa- about 100m. Generally, from October to March is theper. However, the calculation of both sensitive and la- deepening phase of MLD, which is relatively slow andtent heat fluxes is related to the wind speed. In fact,shows a cumulative, gradually deep convection processthe cold westerlies take large amount of heat away following thevolutionof air-sea interaction. At thefrom the sea surface, so the indirect efct of wind can end of March, the seasonal thermocline vanishes. Whilenot be ignored. We will emphasize the contribution in the early Spring (April, May), MLD shoals rapidlyfrom net heat flux, which is known to cause the win-and gives way to the emergence of the seasonal ther-tertime deep convection and deepens the mixed layer mocline. Besides, we find the region over the Kuroshioto a considerable depth. The seasonal variation of the and Kuroshio Extension (25*- 35°N,130*- 170'E) isMLD is crucial to the mode water formation. So, nextthe most distinctive region of MLD variation. This regimewe will analyze the characteristics of the seasonal cycle corresponding to the NPSTMW formation area (Fig.3)40°N(ul)".(Oct)3:30 -20-----.30那- - 120-2102520。1800 10.---1540(May(Aug)35*.303225*230二 1930/JAon30.6-0(Mar)(SCp(ODec3530--”30"80”2(中国煤化工20V们135° 150 165"E 180° 165W 135° 150" 165°E 180* 165'W4fYHCNMHGE180165WFig.3 Seasonal eycle of the net heat flux in the North Pacifie using data from COADS (C1-30 Wm^ 2 )Sensitive region (130*-170'E, 25*- -35'N) for NPSTMW formation process is quantified.138Journal of Ocean University of Qingdao2003, Vol.2, No.2._ JaA>s35*t30°以(FebMeyA吗0|s°f20*|Mar_Sep40^N3530 1200250-19015300。s一e。_130° 140 150 160° 170*E 180° 130 140 150° 160^ I70°E 180° 130" 140 150 160" 170'E 180r 130* 140" 150* 160* 170'E 180Fig.4 Seasonal variation of MLD (alculated by (SST -1C) using XBT dataset in the North Pacific subtropical gyre)is just off the Kuroshio and Kuroshio Extension andsouth of Japan ( Suga and Hanawa, 1995).leans on the Kuroshio Extension Front (KEF, 34.5"N)Next, by analyzing the meridional section in Fig.5(Zhang and Hanawa, 1993). Moreover, the MLD ex-and combining analysis of the seasonal variability ofhibits a two-center pattern, which may be closely re-MLD above, we would elucidate the formation processlated to the anomalous variations of the Kuroshio pathof NPSTMW. We know that based on‘Stommel De_0100150200F25(30350.16iebMay_Auo50|20025030016AarDec5o00号350-”220'N 35"30 25 20N 35中国煤化工业20NFig.5 Depth- laitude sction zonal- averaged between 145*- 170°E,YHCNMHGContours refer to SST (CI -20C ); nude dotted line represents MLD; shaded zone is water mass with PV lessthan 1x10^"*m's'.PAN A.J. et al: Subtropical Mode Water in the Northwestern Pacific139mon' theory, mode water is strongly biased to later 170E. The distribution of Mode Water formed at dif-winter water properties. Fig.5 shows clearly the for- ferent isotherms corresponds well with the March seamation process of low PV water mass, that is, NPST- surface temperature, and this further verifies that modeMW, due to the seasonal variation of MLD along with water is one product of the ventilation process of iso-the seasonal variability of ventilation processes.therms of the permanent thermocline.The Kuroshio Extension Front (KEF, 34.5°N) canDeebe easily detected at about 34°N by the tilted intensiveNon0isotherms. The first information Fig.5 tells us is thatOetNPSTMW exists there the whole year and has distinc-Scptive seasonal variation. In this zonal- averaged meridi-onal section NPSTMW lies south of the KEF and ex-Angtends southward no farther than 24° N with tempera-Julture and spatial characteristics consistent with Fig.2.unHere, we want particularly to make clear the NPST-layMW formation process by considering the seasonalAprvariation of ventilation process of thermocline and thetar |MLD with reference to the seasonal cycle of atmo-spheric forcing.The formation process of NPSTMW could be divid-120 140 160B 180 160°W120" 140” 160'E 180” 160"W .ed into two phases: the first phase may be called the‘ventilation ' stage, which is from October to March. IFig.6 Time longitude section at 30'N, NPSTMW defined asthis stage, the northwestly monsoon begins to strength-PV minimum averaged between (a) 18 -17C,(b) 17-en, and along with the cold, dry airflow there is large16 C isotherms respectivelyamounts of heat losses from the sea surface. Mean-Unitsare 10~*m^'s' with PV<4X 10°m's' hatchedwhile, the 16 -19C isotherms outcrops at the sea sur-lightly and PV<3X 10 'm 's' hatched darkly, data fromface indicating the strengthening of the ventilationLevitus, 1994 a.processes, MLD between 28°- 34' N deepens from onlyabout 50 m in October to more than 300 m in March, 5 Conclusions and Discussioncorrespondingly, water masses with low PV reaches aBy analyzing high. resolution Levitus ( 1994a, b)considerable depth with the deepening mixed layer .dataset we get the climate features of NPSTMW. TheThe second phase features the‘formation' of NPST-long- term mean state NPSTMW lies in the region ofMW. From April to June, there are major changes in25*-35°N,135*- 170'E, south off the Kuroshio andatmospheric forcing, especially the net heat flux. Dur-Kuroshio Extension and extends southward no farthering this period, the heat loss from sea surface reducesthan the STCC (20* N). Its core temperature is be-and even in May the ocean begins to gain heat fromtween 16- 18C with potential density ranging fromthe atmosphere. In the ocean, we find the outcrop-ping zone indexed by 16 - 19 C isotherms narrowed,25- 25.8σg and at a depth of about 80 - 320 m. Thisindicating the weakening of the ventilation process.demonstrates that NPSTMW has a three dimensionalWhat is more important, MLD shoals so quickly,spatial pattern in the western part of the North Pacificfrom about 350 m in March to only 100m or so, thatsubtropical gyre. The above results verify those previ-the remained well mixed water mass with later winterous researches. .properties (low PV) is to be left in the upper oceanIn this paper we aim to make clear the formationand sheltered from further air-sea interaction by theprocess of NPSTMW using XBT and COADS data-emerging seasonal thermocline. This part of low PV sets. Through analying the ventilation processes atwater mass (below 100m), by the mode water defini- the air-sea interface with proper consideration of thetion we adopted, thus forms the NPSTMW .seasonal cycle of atmospheric forcing, such as windConsidering the relatively sparse grid of XBT data-stress and net heat flux, we analyze the seasonal varia-set, we further use Levitus (1994a) data to makeex- tion of MLD and then present a clear picture of theplicit the formation process of NPSTMW from another NPSTMW formation. This provides an observationalapproach. The viewpoint on the formation process ofevidence of the classical ‘ Stommel Demon'. Thus,NPSTMW as low PV water mass on isotherms whichthe formation of NPSTMW could be divided into theoutcrops in wintertime is here tested again, and we' ventilation' and the‘ formation' phase. In the firstgive another evidence on its formatin locality, testi. phase ( October-March), with the strengthening offying to some extent Suga and Hanawa's result of nor中国煤化Ilosses from the sea1995. It is evident that Mode Water is formed locally, sur_being strengthened, .while the warm Mode Water (17- 18C) originating MIYHC N M H Gmaximum valule inin the region 140- 150'E (Fig.6a), and the relatively March between 28*-34"N. In the 'formation ' phasecold NPSTMW (16-17C) in a wider region 140°- (April-May), due to the weakening of ventilation pro-140Journal of Ocean University of Qingdao2003, Vol.2, No.2cess and the emergence of seasonal thermocline, theRuijin for their many enlightening discussions. We al-mixed layer shoals rapidly, the deep part of low PVso gratefully acknowledge the efforts of Jiang Xia andwater mass is no longer subjected to further air-sea in-Dr. Jia Yinglai in data processing and the preparationteraction, and thus forms NPSTMW.of Fig.5. Special thanks are given to Prof. Fu GangWe also show that the warm NPSTMW ( 17 -for the English writing improvement of this paper.18C) origins from 140°- 150°E, while the relativelycold NPSTMW (16 -17C) comes from a wider longi-Referencestude range (140*- 170'E). This locality of mode waterformation corresponds well with the March sea surfaceBingham, F. M,1992. The formation and spreading oftemperature and further testifies that mode water issubtropical mode water in the North Pacific. J. Geophys.one direct product of air- sea interaction at the sea sur-Res., 97: 11177-11 189.face .According to recent research results, there are threeHanawa, K., and I. Hoshino, 1988. Temperature structureand mixed layer in the Kuroshio region over the Izukinds of Mode Water in the subtropical North Pacific.Ridge.J. Mar. Res. 46: 683 -700.They are the NPSTMW, the North Pacific CentralHautala, S. L, and D. H. Roemmich, 1998. SubtropicalMode Water ( CMW) ( Nakamura,1996) and themode water in the Northeast Pacific Basin. J. Geophys.Eastern Subtropical Mode Water (ESMW) (HantaulaRes, 103: 13 055 -13 066.and Roemmich, 1998). Recently, decadal changes ofLevitus,S., and T. P. Boyer, 1994 a. World Ocean Atlas:the equatorial upper ocean thermal structure such asTemperature, Vol. 4. NOAA Atlas NESDIS 4, 177 pp.ENSO events have been addressed in connection withLevitus, S., R. Burgett, and T. P. Boyer, 1994 b. WorldOcean Atlas : Salinity, Vol. 3. NOAA Atlas NESDIS3, 99Mode Waters in the North Pacific subtropical gyre(Liu,1999). Evidences show that the CMW or ES-Liu, z. 1999. Forced planetary wave response in a ther-MW may be transmitted equatorward and thus affectmocline circulation. J. Phys. Oceanogr., 29: 1036- 1055.the thermal structure of the tropical upper ocean. ByMasuzawa, J, 1969. Subtropical Mode Water. Deep-Seacontrast, the NPSTMW has lttle contributions to theRes, 16: 463 -472.decadal variabilty of equatorial upper ocean thermalNakamura, H,1996. A pycnostad on the bottom of theventilated portion in the central subtropical North Pacif-structure for its being mainly confined in the Kuroshioic: Its distributin and formation. J. Oceanogr., 52: 171-Recirculation System. But there are few researches onthe spatial and temporal characteristics of the modeQiu, B.. and Huang, R. x., 1995. Ventilation of the Northwater due to lack of enough data. Therefore, furtherAtlantic and North Pacific: Subduction versus obduction.study on NPSTMW is still very helpful for us to un-J. Phys. Oceanogr.. 235: 2374-2390.derstand the climatic variation in the mid-latitudeStommel, H 1979. Determination of Water mass proper-North Pacific for it may contain the long-term memoryties of water pumped down from the geostrophic flow be-low. Proc. Natl. Acad. Sci. USA, 76: 3051 -3055.of the atmosphere forcing. Meanwhile, the ocean cir-Suga, T, K. Hanawa, and Y. Toba, 1989. Subtropicalculation pattern and thermal structure in the upperMode Water in the 137'E section. J. Phys. Oceanogr, 19:subtropical ocean in the northwestern Pacific is inevi-tably affected by the NPSTMW, especially due to its Suga, T., and K. Hanawa, 1990. The mixed-layer climatol-interaction with the Kuroshio Recirculation System.ogy in the northwestern part of the North Pacific subtrop-The variation of the Kuroshio and the correspondingical gyre and the formation area of Subtropical Mode Wa-climatic variation caused by variations of NPSTMWter. J. Mar. Rer.. 48: 543 -566.Suga, T, and K. Hanawa, 1995. Interannual variations ofstill remain a sophisticated and unsolved problem.Many ises, such as the disipation process in NPST-North Pacific Subtropical Mode Water in the 137 E sec-tion. J. Phys. Oceanogr., 25: 1012-1017.MW, its correlations with K uroshio Recirculation Sys-Suga, T, Y. Taker, and K. Hanawa, 1997. Thermostadtem, the contributions of wind mixing and heat fluxdistribution in the North Pacific subtropical gyre: theto the formation of NPSTMW, and so on, need alsocentral mode water and the subtropical mode water. J.to be studied.Phys. Oceanogr., 27: 140-152Worthington, L. V.,1959. The 18C water in the SargassoSea. Deep-Sea ReRes, 5: 297 -305.Yasuda, T, and K. Hanawa, 1997. Decadal changes in theAcknowledgementsmode waters in the midlatitude North Pacific. J. Phys.The present paper is supported by Free ApplicationOceanogr., 27: 858 -870.(No. 40276009) and NSFC Project for Oversea YoungZhang,R.X., and K. Hanawa, 1993. Features of the wa-ter-mass front in the northwestern North Pacific. J. Geo-Scientist Found (No. 40028605).中国煤化工We give great thanks to Profs. Wang Qi and HuCNMHG

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